1. Field of the Invention
This invention pertains to improved extruded and molded articles, to films, bottles, and the like with improved impact strength and reduced oxygen permeability as a result of the incorporation of a compatibilized barrier resin into such articles, and to methods of preparing such articles.
2. Description of Related Art
The polyolefins combine advantageous properties both technically and commercially. Polypropylene, for example, exhibits greater hardness and stiffness than polyethylene, lower brittleness than standard polystyrene, outstanding resistance to hot water and to chemicals, and very good electrical properties. Thus, the polyolefins and their copolymers have a wide range of applications in the form of containers, moldings, profiles, tubes, films, fibrillated filaments and textile fibers.
Formed polyolefin articles may be prepared by injection or blow-molding. In particular, stretch blowmolding is a mass production process for plastic bottles replacing the glass variety. These are used for oil, wine, spirits, milk, still and CO.sub.2 -containing mineral water, soft drinks and beer. Although the polyolefins possess many desirable properties, as enumerated above, in the particular applications of containers and packaging films, it is desirable to reduce the gas permeability below that normally exhibited by polyolefin films and bottles.
Typically, an extruded film or blown bottle will be comprised of several layers of different polymers, arranged to exploit desirable properties and compensate for detrimental properties. For example, polypropylene offers structural integrity, but is permeable to many gases. A layer of a gas barrier resin, such as a nitrile barrier resin, will reduce the permeability to acceptable levels, even though the barrier resin itself may have undesirable properties, such as sensitivity to liquids. The barrier resin may be sandwiched between layers of the structural polyolefin to isolate the resin, while obtaining the advantages offered by the barrier. In particular, copolymers of ethylene with vinyl alcohol (EVOH) are used in the co-extrusion of packaging materials on account of the outstanding barrier properties against oxygen, nitrogen, carbon dioxide and fragrances imparted by the EVOH. Other polymers, such as poly(vinylidene chloride) (PVDC) may be used as the barrier resin. The selection of barrier resin is well within the purview of the skilled artisan and is not limited to EVOH and PVDC.
One means for the preparation of such desirable packaging materials, such as films and bottles, is the co-extrusion of an outer layer of polyolefin, a layer of barrier resin, and an inner layer of polyolefin.
However, the adhesion between the polyolefin and barrier layers is generally considered to be inadequate for most uses. Therefore, it is necessary to use an adhesive or tie layer between the polyolefin and the barrier layers in order to ensure sufficient mechanical strength to prevent delamination. Typical adhesive materials are copolymers of ethylene, such as Plexar (Quantum Chemical, USI Division).
A representative five-layer bottle may then have an outer polypropylene layer, an adhesive layer, an EVOH barrier layer, a second adhesive layer, and an internal polypropylene layer. Typically, the percentage of adhesive will be about 10% by thickness. A representative construction is approximately 14 mil polypropylene, 1.5 mil adhesive, 4 mil EVOH, 1.5 mil adhesive, and 14 mil polypropylene.
In extrusion blow-molding of bottles of this type, excess material must usually be removed from the finished product. This "scrap" will contain polypropylene, EVOH, and adhesive. For practical purposes, it is desirable to co-extrude this scrap layer with virgin polypropylene, adhesive, and barrier resin forming a six-layer container in which the scrap layer is sandwiched between the outer polypropylene layer and the first tie layer. In this way, the losses attributable to scrap are significantly reduced. This co-extrusion, however, often leads to significant problems with delamination and product quality.
Since it is a critical element of these constructions to have the barrier layer, it is also important that the barrier layer be one which is state-of-the-art in terms of its properties. Although EVOH is a widely used and highly regarded material, it suffers from the deficiency of being susceptible to water. The presence of the polypropylene protects it from water. However, in the construction of these bottles and films, it is still necessary to use the adhesive. The adhesive material adds increased cost and raises significant questions of processibility. It, therefore, is desirable to identify additional adhesive materials which will provide a better balance of properties than that currently available at improved processing efficiencies.